SCR switching power supply

When the thyristor DC stabilized power supply is working normally, it can control a large anode current with a small control current. The thyristor is in an on or off state at the beginning and end of each half cycle of the AC input voltage, so it is called It is a switching regulated power supply. The picture shows a practical circuit of a thyristor ripple-triggered switching regulated power supply. It shapes and amplifies the rectified ripple, and then triggers the thyristor to turn on. When the instantaneous value of the rectified voltage drops to 0V, the thyristor automatically turns off, and the change in the output voltage is used to control the leading edge of the trigger pulse to achieve voltage stabilization. The voltage stabilizing circuit is mainly composed of transistors VT1 ~ VT4, transformer T, silicon controlled thyristor SCR1, etc. When the power is turned on, the rectifier outputs a pulsating DC voltage as shown in waveform ①, which is divided by R2 and R3 and then added to the base of VT4 through C6. The waveform is shown in ②. After VT4 amplifies it, the collector output pulse charges C7 through R8, forming a rising sawtooth wave, which passes through R7 and reaches the base of VT3. The waveform is as shown in ③. At the same time, the error voltage output by the collector of error amplifier VTl is added to the emitter of VT3 through R15. Under the action of the two voltages, the collector of VT3 outputs a 100Hz square wave as shown in ④. The square wave voltage added to the base of VT2 through capacitor C10 becomes a pulse waveform as shown in ⑤. After being amplified by VT2, it is differentiated by the low-frequency pulse transformer T to trigger the control electrode of SCR1. The waveforms are shown in ⑥ and ⑦. When the output voltage changes, the voltages at both ends of the sampling circuit R18~R20 also change. After being amplified by VTl, the pulse width of the square wave output by VT3 is modulated, that is, the pulse front position is changed, so that the length of the conduction time of the thyristor SCR1 is consistent with the output The high and low voltage changes in inverse proportion to achieve the purpose of voltage stabilization. In order to enable the VT4 base to obtain larger pulses, a filter capacitor is added to the output part of the rectifier. In order to ensure that VTl~VT4 can work normally, a filter capacitor C5 is added between R4 and R5. The output of the thyristor has a two-stage output filter. The first stage is a π-shaped LC filter composed of L2, C11, and C12; the second stage is an active filter composed of VT5, VT6, VD10, C13, C14, R22, R23, etc. These two-stage filters can effectively suppress the overshoot voltage and output ripple at the moment of power-on. SCR2, VT7, VD7, VD1l, C16, R24~R27 and fuse FU2 form an overvoltage protection circuit. When the output voltage U0 exceeds the limit value, the voltage is divided by the voltage dividing resistors R26 and R27 and provides sufficient trigger voltage to the thyristor control electrode through VD11 and VT7 to turn on the SCR2, causing the fuse FU2 to blow and protect the load from damage. . The output voltage of this power supply is 110V and the output current is 1A.